Hybrid organic-inorganic perovskite materials have recently become a topic of extreme interest for photovoltaics, with devices based on these materials now reaching power conversion efficiencies of above 15%. Other photoactive crystalline materials are also of substantial current interest.
The key to achieving the highest efficiencies with this material appears to be optimisation of the perovskite or crystalline material film quality. Typically, perovskite and other crystalline films have been deposited by spin-coating a precursor containing necessary components to form the perovskite: a metal halide and an organic component. However, spin-coating is by its very nature a process which can easily result in non-uniform films or films with pinholes. Spin-coating is highly susceptible to microscopic amounts of dust on the substrate, precipitates in the solution, local atmosphere composition and temperature, and human error when depositing. These factors combine to make spin-coating an inherently poorly reproducible process for producing organometal perovskites and other crystalline materials. Moreover, spin-coating is a process which cannot be easily scaled up.
A key breakthrough was achieved by Liu, Snaith et al. (Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398), who describe a high vacuum two-source vapour deposition technique to produce extremely uniform, high quality perovskite films, resulting in the most efficient devices at the time. However, this is a costly process; it requires large quantities of the reagents as well as high temperatures and most importantly, a high vacuum chamber.
Additionally, due to the volatile nature of the organic component employed, it is difficult to control the rate of its deposition, making this process difficult to reproduce between batches.
A second breakthrough was achieved by Burschka et al. (Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gräzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319) who describe a two-step deposition route to form high-quality perovskite films using solution-processing. The metal halide component is spin-coated on a meso-structured substrate and the layer of the metal halide is subsequently dipped in a solution containing the dissolved organic component. The perovskite film forms spontaneously. Recent research has pointed to planar perovskite heterojunction solar cells having the greatest potential to reach the highest efficiencies, rather than those based on a meso-structured (e.g. mesoporous) oxide, and hence the two-step approach was recently adapted for planar solar cells by Liu, Kelly et al. (Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-Temperature Solution Processing Techniques. Nat. Photonics 2013, 1-6). This produced some of the most efficient perovskite solar cells to date. However, the films produced in this way are substantially non-uniform and the surface of the perovskite layer comprises large crystallites of greater than a micron in size. The presence of these crystallites causes high surface roughness. This is likely to be an issue in Willis of reproducibility: if an electrode touches a crystallite of the perovskite penetrating the hole-transport layer it can provide a recombination pathway, lowering device performance and affecting reproducibility. For the best solar cells, an extremely flat perovskite surface would be optimal, allowing only a thin layer of hole transporting material to be employed, and hence minimising resistive losses in that layer.
In light of this, the most recent development for making smooth perovskite films was made by Chen, Yang et al. (Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y.; Li, G.; Yang, Y. Planar Heterojunction Perovskite Solar Cells via Vapor Assisted Solution Process. J. Am. Chem. Soc. 2013, 3-6), who have modified the two-step deposition process, in the planar perovskite solar cell configuration, by replacing the solution-phase organic dipping step with an atmospheric pressure vapour-phase conversion, where the spincoated metal halide is annealed in an atmosphere of the sublimed organic component to convert it to the perovskite. This results in perovskite films of low roughness and high purity. However, this procedure still makes use of spin-coating to form the metal halide initial film, which results in the aforementioned issues with reproducibility and non-uniformity.
Kitazawa et al. (Kitazawa, N.; Yaemponga, D.; Aono, M.; Watanabe, Y. Optical Properties of Organicinorganic Hybrid Films Prepared by the Two-Step Growth Process. J. Lumin. 2009, 129, 1036-1041) describes a process to fabricate nanocrystal-sized (C8H17NH3)PbBr4, using two high vacuum stages. A first vacuum evaporation of a metal halide layer followed by a second vacuum evaporation of an organic halide. A sequential two-step vacuum deposition process is also described in Hu et al. (H. Hu, D. Wang, Y. Zhou, J. Zhang, S. Lv, S. Pang, X. Chen, Z. Liu, N. P. Padture and G. Cui, RSC Adv., 2014, DOI: 10.1039/C4RA03820G).
Methods which make use of vacuum evaporation to form the metal halide layer followed by completion of the perovskite by dipping the vacuum evaporated metal halide layer in a solution of the organic component have not succeeded in producing high quality films. The use of a second solution-based step results in unsuitable perovskite films comprising large crystals and unstable device performance (see Comparative Example 2).
Thus, it is an object of the invention to provide an effective process for the production of perovskite layers. In particular, it is desirable to produce layers of a crystalline material (e.g. perovskite) by a process which is readily reproducible, easy to scale up and produces layers of a crystalline material having a flat surface.